1. Field of the Invention
This invention relates to optical transmission systems, and more particularly, to a technique for suppressing mutually induced interference between closely spaced electrical signals on a Photonic Integrated Circuit (PIC).
2. Description of the Prior Art
Optical transmission media have emerged as the preferred transmission media. Relatively large bandwidth, low cost and ease of implementation are just a few of the reasons which have contributed to making optical media so preferable.
Use of optical transmission media usually requires utilization of electro-optical (E/O) converters to convert an electrical input signal into an optical signal for transmission over an optical medium. For full duplex communications, the optical medium may comprise a separate fiber for each direction, or alternatively, may utilize a single fiber and some multiplexing scheme. One such system is described in "Two Fibers or One? (A Comparison of Two-Fiber and One-Fiber Star Architectures for Fiber-to-the Home Applications)" by Loria Baskerville in Journal of Lightwave Technology, Vol. 7, No. 11, Nov. 1989, and is shown in block diagram form in FIG. 1. The system of FIG. 1 comprises (a) Electro-optical (E/O) converters 101-102, (b) Optical-Electro (O/E) converters 103-104, (c) optical transmission medium 105, (d) directional couplers 106-107, and (e) shielding 108-109, which is not shown in the above reference but is normally utilized in such systems. E/O converter 101 and O/E converter 104 are normally implemented on a single circuit card, as shown in FIG. 1. SImilarly, E/O converter 102 and O/E converter 103 are also normally implemented on a single circuit card. The system of FIG. 1 is intended to provide full duplex communications between end users.
In operation, an electrical input signal arrives at E/O converter 101 and is used to drive a laser having an optical output frequency band centered at F1. The optical output signal is then transmitted over optical medium 105 and received at O/E converter 103, which converts it back to an electrical signal and forwards the electrical signal to an end user equipment. Similarly, E/O converter 102 receives an electrical input signal which it converts to an optical output signal having a frequency band centered at F2. The optical output signal is then transmitted over optical medium 105 from E/O converter 102 to O/E converter 104, where it is converted back to an electrical signal.
Each receiver must contend with interference from two different sources. First, as FIG. 1 shows, light which is transmitted from E/O converter 101 enters optical medium 105 by means of directional coupler 107. However, a portion of this light is reflected back toward O/E converter 104 due to imperfect splicing in the fiber, Rayleigh scattering, imperfections in the directional couplers, or other imperfections in the system. These reflections cause interference in O/E converter 104. This problem has been solved in the prior art by, for example, employing an optical filter between directional coupler 107 and O/E converter 104 which suppresses F1 and passes F2. Alternatively, a wavelength selective directional coupler could be employed. Second, electrical signals entering E/O converter 101 interfere, due to undesirable electromagnetic coupling, with electrical signals exiting O/E converter 104, and cause interference. This second problem has ben solved in the prior art through the use of shielding 108, which serves to attenuate crosstalk between electrical signals exiting O/E converter 104 and electrical signals entering E/O converter 101. E/O converter 102 and O/E converter 103 interfere in a similar manner, with a similar solution as shown in FIG. 1.
The use of shielding 108 and 109 is disadvantageous for several reasons. First, it has recently become possible to fabricate both an E/O converter and an O/E converter on a single chip, known as Photonic Integrated Circuit (PIC). PICs provide significant cost and space savings. Further, alignment of optical waveguides on a PIC can be done photolithographically, rather than manually, thereby making exact alignment easier. Because of space constraints on the PIC, however, it is extremely difficult to implement proper shielding. Further, the shielding used in current systems does not provide the desired amount of attenuation, and thus, there always remains some crosstalk and interference. The shielding problem is extremely severe on a PIC because the O/E converter and the E/O converter are so closely located, and space is not available to apply the shielding. Thus, standard techniques for eliminating crosstalk between the electrical signals work only in the prior art technology, e.g., on a circuit card, and are of little value in state of the art PIC technology. This problem has greatly hindered the development of PICs.
The problem that remains in the prior art is to provide a method of reducing the interference between the electrical signals entering the E/O converter and the electrical signals exiting the O/E converter, where space limitations exist, such as on a PIC.